One of the measures of the molecular weight distribution of the resin is flow ratio (FR), which is the ratio of high load melt index (HLMI or I.sub.21) to melt index (I.sub.5) for a given resin. The melt flow ratio is believed to be an indication of the molecular weight distribution of the polymer, the higher the value, the broader the molecular weight distribution. Resins having relatively low FR values, e.g., of about less than 12, have relatively narrow molecular weight distribution.
The application of the ultimate product will dictate the actual molecular weight distribution of the polymer. For example, LLDPE resins having relatively low FR values produce films of better strength properties than resins with high FR values. By comparison, it has been discovered that in certain high density products used for blown film, high molecular weight and broad molecular weight distribution in the product leads to better processing in terms of bubble stability in high stalk extrusion and in stronger films Many catalyst systems exhibit a tendency to produce resins whose FR values, although initially low, increase with increased concentration of the catalyst activator, also known as a co-catalyst, such as various aluminum alkyls.
It is a significant aspect of the present invention to provide a catalyst composition capable of producing polymers and copolymers having controlled molecular weight distribution and high densities.
It is therefore a primary object of the present invention to provide a high activity catalyst for the polymerization of olefins yielding products of a relatively controllable molecular weight distribution which will depend on the specific composition of the catalyst and the specific cocatalyst used in polymerization.
It is an additional object of the present invention to provide a catalytic process for polymerizing alpha-olefins which yields high molecular weight high density polyethylene of a relatively broad molecular weight distribution at high productivity.